BERLIN—Experts appointed by the German government confirmed on Wednesday that a painting stolen by Nazis and now held in Germany rightfully belongs to the heirs of a late Jewish art dealer, putting the family's efforts to recover the art on the verge of success.

The task force of experts, which had combed a trove of art belonging to the late Cornelius Gurlitt, the son of one of Adolf Hitler's main art dealers, for Nazi loot, said that a Matisse oil painting was stolen from late Jewish art dealer Paul Rosenberg, and...